Tosefta for Menachot 187:7

One who purchases from a merchant tithes from each and every [bread] mold (see Menachot 94a:11), and if [the merchant] was bringing [vegetables from various farmers] and gathering them before [the purchaser], he takes tithes from each and every cucumber and from each and every bunch [of vegetables], and from each and every date. One who purchases from a shopkeeper, and goes back and takes from him a second time, even though he recognizes the barrel [of wine], he may not tithe from this [purchase] on behalf of that [prior purchase (see Dem. 5:6)].